Nanophotonic and nanoplasmonic couplers: Analysis and fabrication.

摘要

Mode mismatch between waveguides of different geometries and propagation mechanisms causes radiation and back reflection, which results in significant loss of optical power. This is considered one of the obstacles that prevents multiple applications of the optical integrated circuits. In this dissertation, we design, fabricate, and experimentally demonstrate four novel photonic couplers that achieve mode matching between hybrid waveguides. These hybrid waveguides include conventional optical waveguides, photonic crystal (PC) waveguides, and plasmonic waveguides. First, we propose a novel method to enhance the coupling efficiency between a dielectric waveguide and a planar PC. This method is based on introducing structural imperfections that cause a change in the mode size and shape inside the taper to match that of the PC line-defect waveguide. These imperfections are introduced by changing the size and position of the inner taper rods. Our results show that introducing the structural imperfections increases the coupling to 96% without affecting the transmission spectrum of the structure. Second, we demonstrate through numerical simulations and experiments that low crosstalk between two crossed line-defect waveguides formed in a square lattice PC structure can be achieved by using a resonant cavity at the intersection area. The PC resonator consists of cubic air-holes in silicon. The theoretical and experimental crosstalk values are approximately -40 dB and -20 dB, respectively. Third, we introduce a novel silicon microring vertical coupler that efficiently couples light into a silicon-on-insulator (SOI) waveguide. A specific mode is excited to match the effective index of the SOI guided mode by oblique incidence. The vertical leakage from the microring forms gradual coupling into the SOI slab. Coupling efficiency up to 91% is demonstrated numerically. The coupler is fabricated and tested to confirm the analytical results. Fourth, we present a novel design, analysis, and fabrication of an ultracompact coupler and a 1 x 2 splitter based on plasmonic waveguides. In addition, we present two nano-scale plasmonic devices: a directional coupler and a Mach-Zehnder interferometer. The devices are embedded between two dielectric waveguides. Our simulation results show a coupling efficiency of 88% for the coupler, 45% for each splitter's branch, 37% for a 2 x 2 directional coupler switch, and above 50% for the proposed designs of the Mach-Zehnder interferometer. In order to confirm the analytical results, the plasmonic air-slot coupler and splitter are fabricated and tested.